1. Field of the Invention
The present invention relates to a method and an apparatus for controlling a fuel-combustion heater for heating a heating medium with combustion heat generated thereby, the fuel-combustion heater being incorporated into a heating medium circuit for heating air by means of heat exchange effected between the heating medium and the air flowing through a duct including a heat exchanger disposed therein and having an air outlet which is openable into a passenger's compartment.
2. Description of the Related Art
Air-conditioning systems for use, for example, on electric vehicles cannot use a heating medium in the form of cooling water which would otherwise be heated by cooling internal combustion engines. For this reason, for example, a vehicular air-conditioning system is known, which is disclosed in Japanese laid-open patent publication No. 8-197937.
The air-conditioning system has a duct for passing air therethrough into a passenger's compartment, wherein the air-conditioning system operates selectively in a refrigerating cycle which involves an air blower for delivering air through the duct into the passenger's compartment, a coolant compressor for compressing and discharging a coolant, a coolant-water heat exchanger for heating hot water through heat exchange between the compressed coolant at a high temperature discharged from the coolant compressor and the hot water, and a coolant evaporator for cooling air with the heat of evaporation of the coolant, and in a hot-water cycle which involves a pump for circulating the hot water heated by the coolant-water heat exchanger and a hot-water heater disposed in the duct for heating air flowing through the duct with the hot water flowing from the coolant-water heat exchanger. The hot-water cycle also involves a fuel-combustion heater, connected in series with the hot-water heater, for heating the hot water with heat generated when a fuel is combusted, in order to obtain a sufficient heating capability, for example, in cold climates.
In the case of such an air-conditioning system, it is known that frost is apt to be formed on the coolant-water heat exchanger when the ambient air temperature is low (not more than 0.degree. C.), and the heat pump cycle effected by the pump suffers decrease in heating ability, in which the heating operation is performed only when the temperature is about -10.degree. C. Therefore, it is desirable that the heating operation for air-conditioning is performed by using the fuel-combustion heater, in a region in which the ambient air temperature is low. In such a case, it is appropriate that the ambient air temperature for starting operation of the fuel-combustion heater is set in the vicinity of -10.degree. C.
When the fuel-combustion heater is used, sufficient heating for air-conditioning is performed without using any heat pump cycle. Therefore, it is possible to achieve energy-saving. On the other hand, for example, the legislation of California in the United States involves the regulation of zero-emission at 4.4.degree. C. or higher temperatures. Therefore, the temperature to start operation of the fuel-combustion heater is generally set within a range of 0.degree. C. to 3.degree. C.
However, in the case of the conventional air-conditioning system described above, the operation to switch on and off the fuel-combustion heater is automatically controlled on the basis of a predetermined ambient air temperature. Therefore, when the ambient air temperature is markedly changed in the vicinity of the ambient air temperature setting to start operation of the fuel-combustion heater, the on/off operation of the fuel-combustion heater is frequently performed. As a result, inconveniences arise, for example, in that the unburned gas is produced from the fuel-combustion heater, and the noise is frequently made upon starting and extinguishment of the fuel-combustion heater.
Further, the operation to switch on and off the fuel-combustion heater is performed regardless of the intention of the passenger. Therefore, the conventional air-conditioning system is not applicable to a case in which it is intended not to produce the exhaust gas, for example, a case in which it is intended to avoid pollution by the exhaust gas, such as in a garage. A problem is also pointed out for the conventional air-conditioning system that it is difficult to drive the vehicle in the zero-emission area.